Qc
Channel deposits (latest Holocene to modern)
clay, silt, sand, and gravel
Unit Qc post-dates the earliest channel dredging and straightening efforts in the Big Nemaha
basin, which occurred primarily from 1904 – 1930 (Rus et al., 2003). These engineering efforts
resulted in measured downcutting of nearly 8 m (26 ft) in the main channel (Rus et al., 2003).
Qc formed during subsequent lateral migration and widening of the channel. The contact
between Qc and adjacent units is a steep to vertical bank with 4 – 10 m (13 – 33 ft) of relief. It is
highly unstable and is subject to bank failure, especially during large floods. Channel deposits
are typically 2 – 3 m (7 – 10 ft) thick near the river edge and thicken distally to 4 – 6 m (13 – 20
ft). They exhibit scroll topography and are unvegetated near the river edge, becoming partly
vegetated with grass, brush, and trees on older scrolls at slightly higher elevations. Proximal
areas of Qc are frequently flooded.
Qal
Alluvium of tributaries of the Big Nemaha River (Holocene to modern)
silty clay, silt, and very fine to fine sand, with basal gravels and coarse sands derived from local
bedrock or till
Unit Qal includes the alluvium of named and unnamed tributaries of the Big Nemaha River.
These deposits vary in thickness from a feather edge to a maximum observed thickness of 12 m
(40 ft).
Qan
Alluvium of the North Fork Big Nemaha River (Holocene)
silty clay, silt, and very fine to fine sand, with basal gravels and coarse sands derived from local
bedrock or till
Unit Qan includes alluvium of the North Fork Big Nemaha River. This unit may include some
low-relief features such as terraces and small alluvial fans, but data are insufficient to map these
units. Unit Qan is as much as 12 – 15 m (40 – 50 ft) thick. The contact between unit Qan and
Qc is a steep bank 4 – 10 m (13 – 33 ft) high and is subject to failure, especially during floods.
Gravel and coarse sand typically form the lowermost 1.5 – 3 m (5 – 10 ft) of this unit.
Qsw
Slope wash and colluvium (late Holocene)
clayey silt, sandy silt, sand, and gravel
Unit Qsw includes slope materials derived from either 1) unconsolidated loess, till, or sand
(slope wash), 2) bedrock (colluvium), or 3) a combination of both unconsolidated materials and
bedrock. Qsw exists as thin mantles of sediment along valley margins and in the headwater
portions of ephemeral drainages.
Qp
Peoria Loess (late Pleistocene)
slightly clayey silt
Unit Qp is massive to faintly laminated silt and slightly clayey silt. It is brown to yellowish
brown, except for where surface soil is developed, where it is black, dark gray, or dark grayish
brown. Qp is present only on the tops of ridges and on some gently sloping, southeast-facing
surfaces in the east-central part of the mapped area. Qp ranges in thickness from a feather edge
to 2 m (6.5 ft) in test hole 3-TR-14. Small, isolated areas of Qp (e.g. near 5-TR-14) may be less
than 0.3 m (1 ft) thick and present only within the upper horizon of the modern soil. Qp
primarily overlies till, but it may also be present locally atop bedrock units in the southern part of
the mapped area.
Qt
Till, slope wash derived from till, and stratified units of till, silty clay, silt, and sand
(Pleistocene [pre-Illinoian])
clay to boulder diamicton
Unit Qt is a matrix-supported, calcareous diamicton containing pebbles, cobbles, and boulders of
limestone, pink (Sioux) quartzite, and igneous and metamorphic rocks. It is dark gray to gray at
depth. The upper part is weathered to yellowish brown. In some areas Qt contains interstratified
silts and sands and is probably gradational with unit Qs (e.g. measured section 3129A). Qt may
be as much as 61 m (200 ft) thick in the northern part of the mapped area where it buries the
underlying bedrock paleovalley, but is generally less than 12 m (40 ft) thick on the bedrock high
south of the paleovalley.
Qs
Sands and silts within or below till (Pleistocene)
silt and very fine to fine sand
Unit Qs exists in scattered, isolated outcrops mainly in the northwest part of the mapped area.
Sands are generally well sorted. Most areas mapped as Qs are probably part of a single large
body of silt and sand beneath or within till, as observed in test hole 3-TR-14. Qs may be
laterally equivalent to unit Qt. For example, large bodies of Qs are observed in close proximity
to units of Qt containing interstratified sands similar to those of unit Qs (i.e. measured section
3129A and test hole 1-TR-14).
Pz
Zeandale Formation (Pennsylvanian)
limestone and shale
Unit *z exists at the surface only in small areas in the southwest corner of the mapped area
where the resistive Tarkio Limestone Member forms discontinuous benches on hillslopes. The
contact between units *z and *w is at the base of this limestone. Unit *z is as much as 8.4 m
(27.5 ft) thick in the subsurface in test hole 7-B-67. Only the lower 3 m (10 ft) is exposed in
outcrop.
Pw
Willard Formation (Pennsylvanian)
non-calcareous to slightly calcareous shale
Unit *w consists of soft, erodible shale that forms gentle slopes. Outcropping areas of the entire
10 m (32.8 ft) thickness of *w are in the southwest part of the mapped area where unit *z is
present. In other areas, the upper 2 – 3 m (7 – 10 ft) of unit *w is covered by Qt or Qp.
Pbe Burlingame Formation, Soldier Creek Shale, Wakarusa Limestone, Auburn
Formation, and Emporia Formation (Pennsylvanian)
interbedded limestone, calcareous to non-calcareous shale, and claystone
Unit *be comprises several stratigraphic units of varying lithologies. The total thickness of these
units is 16.5 m (54 ft) in test hole 5-TR-14. Resistant limestone units form three distinct benches
on hillslopes. Areas mapped as *be are generally rugged lands covered by forest or grass.
Ps
Scranton Formation (Pennsylvanian)
noncalcareous shale with minor siltstone, very fine- to fine-grained sandstone, and thin
limestone
Unit *s is predominantly very dark gray to dark gray, noncalcareous shale and is 34 m (112 ft)
thick. Minor amounts of laminated siltstone and heterolithic shale/sandstone are present locally.
At least two thin to medium beds of limestone and shaly limestone are present near the middle of
*s. Unit *s contains the White Cloud Sandstone Member in test hole 4-TR-14 and in nearby
outcrops. The White Cloud is 33.5 m (110 ft) thick and consists of fine-grained, micaceous,
carbonaceous, and cross stratified sandstone. The lower 1.4 m (6.2 ft) is a limestone- and shaleclast conglomerate. The White Cloud thins eastward to 2 m (7 ft) over a distance of 500 m (1640
ft), where it directly underlies the Burlingame Formation. The lower surface of the White Cloud,
therefore, must have at least 33.5 m (110 feet) of vertical relief and it must truncate nearly the
entire thickness of the underlying shale unit. Limestone beds overlying the White Cloud are as
much as 6 m (20 ft) higher in elevation compared to locations where unit *s is predominantly
shale, suggesting that surrounding shales were compacted more during burial relative to the
sandstone body.
F
Artificial fill
dams, lagoons, or other engineered ground
P
Pit, mine, or quarry (active or abandoned)
excavations, rubble, or disturbed ground related to mining operations
W
Water